There is a global decline in the number of children born to each woman;
each woman has half as many children as her counterpart did in 1972. In Canada (2006 data) there were about 1.5 births per woman; 2.1 would be required to maintain the population.

Why?

The declines are only partially due to contraception, which was a mechanism that facilitated the underlying causes. The drivers included changing values; the emancipation of women and their increasing participation in the labour force; the high costs for child care and education; couples with fewer children enjoying opportunities for travel and leisure and an improved standard of living. (See Shah I. Eur J Contraception and Reproductive Health Care 1997; 2(1): 53-61). The instability of many partnerships was also a factor.

Upstream of these factors were economic pressures, industrialization and
urbanization. Changing labour markets are demanding increasingly educated and skilled
workers, so the cost to parents of raising and educating numerous children becomes
prohibitive. It also requires longer time in education, delaying childbirth among more educated people. As small-scale, family-run businesses decline, children are less
economically useful to their parents.

What is the impact?

Initially, the declining birth rates will bring economic benefits:

Parents have more time to work and earn, increasing spending power, creating
demand which stimulates production and jobs: the economy may expand.

The appeal of radicalism and social tensions may also decline as
young adults make up less of the population and if opportunities rise for their
participation in the labour force.

Likewise, the aging population makes
wars seem less likely: there are fewer young soldiers to fight.
Mothers are likely to oppose the idea of losing their only son in a war.

In industrial
countries, the military also has to devote more of its budget to pensions,
leaving less to purchase weapons. (See "The global baby bust" by Phillip Longman, Foreign Affairs 2004, vol 83(3), pp 64-79).

Fewer children initially means a reduction in the dependency ratios, but
this is later reversed as the numbers of elderly people rise.

Subsequently, things will not be so good:

Health
care for elderly people costs more than education for the young, so population aging will eventually strain government budgets for health care and pension plans, and with fewer workers to pay the taxes.

In several European countries pensions and health care
costs are likely to consume 30% of all national output.

Health may actually
decline due to the lifestyle changes of the urban environment, with its
greater stresses and sedentary lifestyles. At the same time, retirement means that income tax revenues required to
fund these programs decline; the economy shrinks.

The same goes for
employers: General Motors has 2.5 people on its pension rolls for each
active worker and paying for their pension plan adds $1,800 to the price of every
new car they make.

Perhaps an aging workforce leads to less innovation.

Capital investment may decline as elderly people cash in their investments
to cover daily costs of living.

The effects of these transitions are likely to be especially harsh for
developing economies. European countries at least had the opportunity to
grow rich before they grew old; Asian countries may grow old before they can grow rich. It may prove more difficult for developing countries to kick-start their
economies once the aging populations of rich countries reduce their demand
for consumer goods. By 2010, Ontario school enrolment dropped by 100,000 compared to 2002.

Another effect is the increase in births to older women: in Canada in 1998 the fertility rate was around 30 births per 1000 women aged 35-39. This rose to 45 births in 2007. (Source: CIHI report, "In due time", September 2011)

2. Nice to Know

What can we do?

Among the potential answers to the "baby bust", immigration can work in
the short term, but most immigrants are already in their adult years and
will themselves become old relatively soon. High ratios of immigrants can
also create social tensions. And where would the immigrants come from?
Many potential sources are themselves also in a state of demographic
transition to lower birth rates.

It is possible that the birth rates may rise again. One candidate to
lead this would seem to be groups that are less influenced by the market
pressures that led to the decline in birth rates. For example, religious groups
often emphasize procreation: 90 children are born to every
1,000 women in Utah, compared to 49 in Vermont. Perhaps government policies
that offer financial support to parents will be influential. Maybe the world will become more religious!

World Data:

In Bangladesh fertility rates have fallen from 6.8 children per woman to 2.7 in 2010; In China the decline was from 5.6 to 1.7; Brazil went from 6.1 to 1.8. In Brazil, economic growth has given women the chance to work, while popular local t.v. soap operas (novelas) portray working women with small families.

Nigeria declined less: from 6.5 in 1955 to 5.2, and Yemen declined from 8.3 to 5.1.

The rate of growth of the world population has halved since the 1960s; the number added to the population has fallen every year for over 20 years. The drop in fertility has been driven mainly by economic development, female emancipation, education, contraception and by the defeat of child mortality: women can now actually plan their families.

About 59 countries (comprising about 44% of the world’s population) are
not producing enough children to maintain their population.

Demographers predict that the world population will peak at about 9
billion by 2070 and then start to contract. The population of Russia, for
example, is currently contracting by about 750,000 per year and Japan’s
population is expected to fall by about one third during the next 50 years.
Fertility in the Middle East is falling especially fast. (See "The global baby bust" by Phillip Longman, Foreign Affairs 2004, vol 83(3), pp 64-79).

in 2010, Europe was the continent with the lowest average "total fertility rate" (TFR), at 1.5 children per woman. Fertility decline was especially rapid in Southern Europe: Spain and Italy have 1.2 children per woman.

Nerd's Corner: Is a one-child policy necessary?

Philip Bowring, a Hong Kong based journalist has written forcefully against the Chinese one-child policy ("Lurid stories of forced abortions...a policy which has given the Communist Party outrageous power..." etc). He makes the following arguments:
1. Singapore and Hong Kong saw steep declines in birth rates with no such policy (Singapore went from 6 births per woman in 1950 to 1.8 in 1980).
2. Although these are atypical city-states, other, more rural countries have also seen declines: Thailand and South Korea.
3. Thailand made this possible "simply by making contraception easily available in all locations and being unabashed about sex." (Can it be that simple?)
4. Bowring argued that abandoning the one-child policy may not make much difference in China: there are now relatively few women of child-bearing age in rural areas thanks to widespread urban migration. And urban areas will follow the international trend: Shanghai currently allows couples a second child but has a birth rate of 0.7 per woman, one of the lowest in the world.
5. The sex imbalance in China, combined with growing education and working opoortunities for women, is far too large for easy solution. China is too large for immigration to have an effect.
Bowring argues that the Chinese government will have to use public funds to lower the opportunity cost of having children - e.g., providing job protection for mothers, tax incentives and free education for the children.
(Hong Kong Sunday Morning Post, July 29, 2012)